The explosion in Internet usage in recent years has greatly accelerated the widespread use of TCP/IP protocols suite as well as a dramatic increase in packet data traffic. With the ever rising demand of mobility and M-commerce, it is logical to extend such protocols into the wireless world. However, the existing 2G wireless systems. Examples of such systems are Global System for Mobile Communication (GSM), IS-95, IS-136 and the like, which are primarily built for traditional voice communications. Such circuit-switched networks are not well-suited for sending data. For instance, the data rate supported in GSM is only up to 14.4 kbit/s. Although 3G technologies can handle packet data more effectively and may achieve a peak rate of 2 Mbit/s (under favorable conditions), they have to accommodate circuit-switched data at the same time. Therefore, there should still be room for improvement for packet data transmission.
Meanwhile the growing popularity of Transmission Control Protocol/Internet Protocol(TCP/IP) leads one to seriously consider the possibility of a new generation of wireless services running solely on TCP/IP protocols that are capable of supporting both voice and data communications. That is, using voice over IP (VoIP) telephony, speech signals are transported as packet data and integrated together with other packet data in the network. Such a packet-based network in the long term may well replace the traditional circuit-switched networks, thus resulting in a unified wired and wireless IP networks for both voice and data, with many advantages like economics of scale, seamless services, global standardization, and the like.
It is well-known that voice and data transmission have different requirements. One fundamental difference between wireless voice and data communications is their behavior in a time-varying Radio Frequency (RF) channel. Voice may only accept a latency of up to about 100 msec,; however, data may bear a much larger value. Voice transmission also requires a certain minimum signal-to-noise (SNR) ratio to be met a good channel quality would not necessary improve the speech quality, but a poor channel may cause serious deterioration. On the other hand data is more flexible, data flow may be increased in good channels to boost the throughput, and, conversely, it may be reduced in poor conditions in exchange for a lower bit error rate (BER).
Capitalized on these differences the idea of link adaptation or adaptive modulation, which is the technique adopted in Enhanced Data for GSM Evolution (EDGE) to push the maximum data rate to beyond 384 kbit/s, has emerged recently. In this concept the modulation constellation, coding scheme, transmitter power, transmission rate, and the like, are adapted to the fading channel quality. When the channel is good, a high order modulation with little or no coding is used, conversely when the channel is bad a low order robust modulation is chosen. Several camps of academic researchers have contributed to this subject. Via theoretical and simulation studies, they showed that data throughput and system capacity may be improved or optimized while maintaining an acceptable bit error performance.
Typically, the channel quality is assessed by the instantaneous signal-to-noise (SNR) ratio, which is divided into a number of fading regions, with each region mapping into a particular modulation scheme. Thus one basic issue in adaptive modulation is to determine the region boundaries or switching threshold, i.e. when to switch between different modulation schemes. A common method of setting the thresholds to the signal-to-noise ratio (SNR) required to achieve the target Bit Error Rate (BER) for the specific modulation scheme under additive white Gaussian noise (AWGN) has been shown in the art. While this maintains a target BER, this does not optimize the data throughput which is probably a more important concern for data transmission. In Nokia's (Finland and Irving, Tex.) joint “1XTREME proposal” with other companies to 3GPP2, the switching thresholds are derived from steady state throughput curves of the individual modulation schemes. This increases the throughput relative to the previous method but still is not optimal. For packet data transmission in a time-varying channel, what would be desirable is an on-line adaptive scheme that can adjust the switching thresholds dynamically to maximize the throughput.